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Proportional Symbol Map

A proportional symbol map is a type of map that includes symbols that change in size based on the value they represent.

Proportional symbol maps are typically used to show counts or totals and can be used to visualize classed or unclassed data. When using classed data, the map becomes a type of proportional symbol map called a graduated symbol map. The difference between these two types of maps is that graduated symbol maps use range grading, whereas proportional symbol maps use absolute scale or apparent magnitude scaling.

Requirements

Always

Consider the relative size of all symbols. If the symbols overlap, is the map still legible?

Consider the orientation of the symbol. If the symbol is not a circle, ensure the symbol is oriented properly.

Always choose a method for grouping your data carefully when using a graduated symbol map.

The Natural Breaks classification method groups data based on gaps in the data.

The Equal Interval method divides the data range into equal intervals. This method works best when data are evently distributed. If the distribution is uneven, the predominant values will dominate the map.

The Quantile method partitions the data so that every group has an equal number or nearly equal number of values. This can give a choropleth map an even, aesthetically pleasing color distribution.

Subject matter expertise can be used to place data into groups.

Recommendations

Recommended

It is recommend that you limit the number of classes to five or fewer categories.

Fewer symbols on the map are more easily distinguished by the eye and will make trends in the data easier to see.

It is recommended that proportional symbol maps of the United States use an Albers equal-area projection, including insets of Alaska, Hawaii, and Puerto Rico, if applicable.

Not Recommended

It is not recommended to have symbols on the map overlap. This can sometimes be appropriate depending on the type of symbol and scale of the map.